Recent requirements have led to controlling of operations of safety devices in accordance with a weight of an occupant seated on a seat for a vehicle, for the purpose of enhancing performances of safety devices, such as a seat belt, an air bag, and so on.
WO2006/011597 (hereinafter, referred to as reference 1) discloses therein an occupant weight measurement device. The occupant weight measurement device includes: a left and right pair of fixed lower rails fixed to a vehicle; a pair of movable upper rails provided so as to be movable in backward and forward directions on the respective fixed lower rails; a load sensor mounted on each movable upper rail and having a rod extending vertically upwardly; and a rectangular frame mounted above the pair of movable upper rails and formed with installation holes. The rods of the load sensors are vertically upwardly inserted into the installation holes of the rectangular frame, respectively and, thus the rectangular frame is mounted on the load sensors.
As described above, because the rectangular frame, which is shaped in advance, is mounted on the load sensors with the rods of the load sensors vertically upwardly inserted into the installation holes of the rectangular frame, occurrences of initial strain and initial load are restrained. As a result, it is possible to measure a weight of an occupant more precisely than an occupant weight measurement device, by which components for the rectangular frame are mounted on the load sensors and then assembled to the rectangular frame.
In addition, a strain sensor is disclosed in Japanese Patent No. 2003-83707A (hereinafter, referred to as reference 2). According to the strain sensor of the reference 2, a detecting member is press-fitted into a detecting hole of a sensor substrate (strain member), and an external force (an external force for generating a strain to be detected) is applied to an end portion of the detecting member, which the sensor substrate (strain member) is in contact with. In the mean time, a first fixing member is press-fitted into a first fixing hole of the sensor substrate (strain member) and a second fixing member is press-fitted into a second fixing hole of the sensor substrate. The strain sensor is configured to apply component force, relative to the external force, to end portions of the first and second fixing members respectively, end portions which the sensor substrate (strain member) makes contact with.
According to a detecting device with the above-described structure, positions, where the sensor substrate (strain member) and the end portions of the detecting member, the first fixing member, and of the second fixing member respectively make contact with, are not dislocated. Therefore, a bending stress applied to a strain detecting element (strain gauge) is stabilized and an output precision of the strain sensor is increased.
In the aforementioned occupant weight measurement device according to the reference 1, load, which corresponds to a weight of an occupant, is converted into an electrical signal by a strain gauge at a sensing portion of the load sensor. The electrical signal, however, is influenced by even slight variations in electrical resistance. Therefore, in general, an amplifier having an electronic circuit is arranged in parallel, so that the variations in electrical resistance are amplified to be read. Meanwhile, various electromagnetic waves are induced by various in-vehicle equipments, such as drivers, lighting equipment, and so on, and influence on complex electronic circuits, so that an erroneous measurement and improper operations may occur. In light of the foregoing, in general, the amplifier is housed in an amplifier case made of metal such as aluminum, and the amplifier and the amplifier case are both connected to earth, thereby shielding electromagnetic waves (noise). However, when the aluminum-made amplifier case is housed in the sensing portion, the sensing portion becomes large and requires more space.
Further, according to the strain sensor disclosed in reference 2, the sensor substrate (strain member) is immovably fixed by the first and second fixing members. Therefore, when the external force is applied between the both first and second fixing members, the sensor substrate receives a tensile force in directions of the first and second fixing members and is prevented from being flexibly deformed. In addition, the sensor substrate is configured so that the component force is generated at the end portions where the fixing members make contact with the sensor substrate (strain member), thus preventing the sensor substrate from being flexibly deformed. Accordingly, sensitivity for detecting the load may be reduced.
A need thus exists for an occupant load detecting device which is not susceptible to the drawback mentioned above.